1. Field of the Invention
The present invention relates to a construction of a shutter of an image pick up apparatus such as a digital camera having a function of displaying a visual image picked up by an image pick up element on a visual image display device of liquid crystal or the like.
2. Related Background Art
Various kinds of shutter devices as follows are used in conventional digital cameras each having a function of displaying an object image picked up by an image pick up element on a monitor such as an LCD (Liquid Crystal Display) as a finder image for observing an object.
A first prior art example is a conventional shutter device used in a silver camera. As is well known, the conventional shutter device holds curtain groups divided into two or more with two arms to be rotatable to form a parallel link. This shutter device has a front curtain which shifts a shutter opening to an open state from a shielded state at a time of exposure, and a rear curtain which shifts the shutter opening to the shielded state from the open state at the time of exposure. The front curtain is connected to a front driving member having a driving source for running a blade such as a spring, and an armature which is suction-held by a magnet. Then, in a preparation state for image pick up and recording, the armature is brought into contact with a yoke suction surface of a magnet for the front curtain by a charge lever, and the front curtain is held in a state in which it shields the shutter opening. The rear curtain is connected to a rear driving member of the same construction as that of the front curtain. In the preparation state for image pick up and recording, the armature is brought into contact with a yoke suction surface of a magnet for the rear curtain by the charge lever, and the rear curtain is held in a state in which it opens the shutter opening. After release, the charge lever retreats from the curtain holding position, and the front curtain and the rear curtain are held in a start position by a magnetic force of the energized magnet. Then, energization for the magnet for the front curtain is cut off first, the front curtain releases the shutter opening, then, predetermined seconds later, energization for the magnet for the rear curtain is cut off, and the rear curtain shields the shutter opening. Exposure time is thus formed and the screen is exposed. After completion of running, the front curtain and the rear curtain move to the image pick up preparation position by the charge lever, and wait for the next photographing. The first prior example is constructed as above.
A second prior art is a focal plane-shutter device for a digital camera disclosed in Japanese Patent Application Laid-Open No. 2001-215555 and Japanese Patent Application Laid-Open No. 2002-23220. When power is applied to a camera including a shutter device of this kind, a front curtain is in the state to open a shutter opening, so that an object can be normally observed with a monitor such as an LCD.
A third prior art example is a shutter device disclosed in Japanese Patent Application Laid-Open No. S57-17935, in which when running of a front curtain is not detected by a detecting device which detects running of the front curtain, running of the rear curtain is inhibited by an inhibiting member.
In the above described first prior art, on the occasion of observing an object with a monitor such as an LCD, the object image is caught by an image pick up element, and therefore, the front curtain needs to fully open the shutter opening. In the shutter device of the first prior art example, the armature is sucked by energizing the coil of the magnet for the front curtain, and the front curtain is held in the running start position. When a start signal to operate the front curtain to open the shutter opening is inputted, energization for the coil of the magnet for the front curtain is cut off, the armature is released from suction to the yoke, and the suction surface of the above described magnet is in the uncovered state. When the rear curtain is held in the running start position, the armature is sucked by energizing the coil of the magnet for the rear curtain. When the object is observed with the monitor such as an LCD, the object image is picked up by the image pick up element, and therefore, in the state in which the front curtain fully opens the shutter opening, the charge lever which mechanically holds both the front curtain and the rear curtain in the overcharge state returns to the retreat position. Therefore, the coil of the magnet for the rear curtain has to keep energized so that the rear curtain does not close the shutter opening. Therefore, there is the disadvantage of wastefully consuming electric power. Further, when the time to observe the object with the monitor such as an LCD is long, the temperature of the coil of the magnet for the rear curtain which is kept energized becomes considerably high. When the photographer picks up an image in this state, there is the disadvantage of lowering control accuracy of the exposure time at a high-speed exposure time due to instability of the operation characteristics of the magnet for the rear curtain.
Besides, the above described second prior art has a high risk of dust attaching to a low path filter and the like disposed at a front surface of the image pick up element, because when it is used for a system camera capable of replacing a lens, the shutter opening is kept open at the time of replacement of the lens. When dust attaches to the low path filter and the like, quality of the picked up image is degraded. It is conceivable that the image pick up element is irradiated with sun light for a long time when the camera is left in the bright outdoors or the like without a lens cap or the like put on, which easily causes deterioration such as color fading of the color filter provided at the image pick up element.
Besides, in the above described third prior art example, movement of the inhibiting member which inhibits running of the rear curtain uniquely links to the movement of the detecting device which detects running of the front curtain, and therefore, only the position for releasing the running inhibition of the rear curtain can be taken in the state in which running of the front curtain is completed.
Therefore, it is conceivable that the shutter devices are susceptible to improvement to solve these problems.